Physical Sciences Division Research Highlights

September 2006

Playing Keep-away with Sulfate

Scientists provide molecular view of sulfate salt dissolving in water

Lai-Sheng Wang

Researchers at the Pacific Northwest National Laboratory and the Czech Republic's Academy of Sciences have just determined how water molecules pry apart a complex molecule. The research team, which has just published a paper on their discoveries, studied sodium sulfate ions. These ions, as the salt sodium sulfate, appear in natural deposits, laundry detergent and other common household products, as well as in atmospheric studies of deteriorating air quality.

"We obtained a vivid picture of the initial processes of how water molecules start to 'desolve' a sodium sulfate molecular ion," said PNNL affiliate senior chief research scientist Lai-Sheng Wang, who is also a professor of Physics at Washington State University, Tri-Cities. "This study is some of the earliest work done concerning more complex salts."

Water molecules wrap around sulfate, pushing sodium away
Dissolution is a common daily experience, such as when one puts sugar or table salt in water. To understand what happens when sodium sulfate meets water, the team generated water clusters of the complex salt and performed experiments using photoelectron spectroscopy at the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy national scientific user facility located at PNNL in Richland, Wash.

By performing sophisticated calculations and comparing with experimental observations, the scientists obtained a clear image of the first steps of dissolution. They found that three water molecules are enough to pry apart the sodium and sulfate, by forming a ring between the two ions.

The computed structure of NaSO4(H2O)3 next to its photoelectron spectrum. The red atoms are oxygen, the golden atom is sulfur, the grey one is sodium, and the white atoms are hydrogen.

Future challenges look at temperature and more water
The researchers hope to study the dissolution process further. "In this study, we were only able to make clusters with up to four water molecules," said Wang. Future studies may look at what happens when more water molecules get into the act, furthering the understanding of the molecular games of dissolution.

In this study, the work was performed at room temperature. Because dissolution rates and reactions depend on the temperature, future research may look at controlling the temperatures of the clusters.

The U.S. Department of Energy's Office of Basic Energy Sciences funded the experimental research. The Czech Ministry of Education and U.S. National Science Foundation funded the calculations.